If the Merge Group is a permanent (non-shared) merge group, then the logical volume will have a fixed SCSI ID. If it is a shared merge group, and there is more than one shared merge group in the system, the SCSI ID will vary depending on when it is made available to the system.
Logical Volumes are assigned in the order they're made available. So, if you have more than one, your SCSI IDs might be different depending on the order you HA system has made them available to the system.
This means that in an ActiveActive arrangement, you cannot rely on knowing what the SCSI ID is, and must rely on volume labels, or uuids, or logical volume managers to perform the association between the volume and the mount point for you.
You can do this with the Filesystem resource agent by using the -L or -P flags in the resource specification.